1. Field of the Invention
The present invention relates to an apparatus that uses high-frequency electrical signals to modulate optical signals, and particularly to an optical frequency converter using reciprocating modulation, that exhibits substantially the same effects as the case where optical modulation is conducted using a high-frequency signal having a frequency that is an integer of times higher than that of a high-frequency electrical signal.
2. Description of the Prior Art
There have heretofore been known apparatus for converting the frequency of light input using various methods. One of the methods is to project two kinds of laser beams onto a non-linear optical crystal to mix the beams. This method has already been known and is used when the frequency of a laser beam is to be doubled. Another method is to use a mode-locked laser. This method includes a process of generating optical pulses by equipping a laser resonator with an optical modulator, an isolator and a Fabry-Perot etalon, which process is also known as a process for generating a sideband having a high-order frequency fp that is Km times the phase modulation frequency fin (fp=Kmxc2x7fm). Still another method is to convert an optical frequency by modulating a laser beam using a high-frequency signal to generate a sideband. The present invention is similar to the third-mentioned method and will be described hereinafter.
Optical modulation by highrequency signals is generally accomplished by inputting an optical carrier wave and a high-frequency electrical signal to an optical modulator and performing intensity modulation or phase modulation or other such modulation. In the case of this method, in order to obtain a sideband having a frequency that is at least as high as that of the applied high-frequency electrical signal, the high-frequency electrical signal is multiplied to form an even higher-frequency signal that is used for the optical modulation. However, even when the high-frequency electrical signal is thus multiplied, the maximum modulation frequency is determined by the upper limit of the electrical signal. Thus, in terms of frequency, an electrical signal that is multiplied or amplified or other such signal is limited by the maximum characteristic of the electrical circuit. However, there are cases where a high frequency exceeding the limited frequency is demanded as a modulation frequency. There is therefore a need to satisfy this demand.
There have been reports of attempts to produce a sideband with a frequency higher than that of the applied high-frequency signal, using phase modulation with a high modulation index. One Reference (xe2x80x9cGeneration of Ultrasort Optical Pulses Using Domain-Inverted External Phase Modulator,xe2x80x9d by T. Kobayashi, OYO BUTURI, Vol. 67 No. 9 (1998), pp. 1056-1060) describes applying a 16.26 GHz electrical signal to an optical modulator having a waveguide formed of an electro-optical crystal of LiTaO3 on which a stripline resonator was disposed. With a modulation index set at 87 radian, the spectral bandwidth was around 2.9 THz.
In addition, U.S. Pat. No. 5,040,865 discloses a method for producing a high-frequency electrical signal by modulating monochromatic light with a high-frequency electrical signal using a modulator having non-linear characteristics to generate a high-order sideband and using a photodetector to detect an optical signal of the sideband. This also discloses a method comprising the steps of obtaining a first high-frequency electrical signal by the method mentioned above using a first modulator and applying the first high-frequency electrical signal to a second modulator to modulate it with a second high-frequency electrical signal. However, since the latter method uses an electrical signal obtained by multiplying a given high-frequency electrical signal, it is subject to restriction in relation to the frequency of an electrical circuit.
In order to generate a high-order sideband, it is necessary to obtain a high modulation index as was done in the phase modulation configuration of the aforementioned Reference. To obtain a high modulation index, a high-frequency electrical signal having large amplitude has to be used. In order to increase the amplitude of the modulation signal, a stripline resonator is used as the modulator electrodes, making it difficult to change the modulation frequency. In addition, it can be avoided to use a resonator as the modulation electrodes by using ordinary electrodes not having the resonance characteristic and amplifying a high-frequency electrical signal. Therefore, it is easy to think of a configuration making it possible to readily change the modulation frequency. However, it is well known that the amplifier using the modulation signal would place an upper frequency limit on the high-frequency electrical signal used as the modulation signal.
In view of the above, an object of the present invention is to provide an optical frequency converter using reciprocating modulation to obtain a high-order sideband even when modulation is performed using a high-frequency electrical signal having a smaller amplitude than in the conventional configuration that uses, as the principle of operation, the phase modulation adopting a high modulation index.
To attain the above object, the present invention provides an optical frequency converter using reciprocating modulation, comprising: means that, taking n as a predetermined integer of 1 or more, modulates light of a predetermined frequency to produce an nth order sideband group thereof, means that modulates the nth order sideband group to produce an (n+1)th order sideband group; and means that selects a specific sideband from a plurality of sideband groups. Here, xe2x80x9cnth order sidebandxe2x80x9d refers to a sideband that is frequency-separated by n-times the modulation frequency from the carrier wave, and the xe2x80x9cnth order sideband groupxe2x80x9d refers to two sidebands located symmetrically with respect to the carrier wave.
The optical frequency converter of the present invention also includes reflection means used to fold a light path for the light of the predetermined frequency and the modulated light. It can further includes one or more modulation means for modulating the light of the predetermined frequency, to one of which a sideband group of a different order is input, whereby the number of modulators used is decreased to reduce the cost of the converter.
The converter can further include first reflection means that prior to modulation passes light of a predetermined frequency and reflects part of light of other frequencies and second reflection means that with respect to a predetermined integer n of 1 or more, passes (n+1)th sidebands and reflects part of other light. It can further include a laser light source and an optical modulator, wherein the first reflection means comprises a first narrow-bandpass filter and the second reflection means comprises a second narrow-bandpass filter. It can further include a phase modulator for further modulating the signal. Moreover, it can further include means for converting part of the optical output into electrical signals and means for re-inputting the electrical signals to the modulation signal of the phase modulator.
The above and other objects, further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.